I use a GPS speedo daily and find it works very well even among city canyons.
How long ago did you work on that project? I wonder if modern units perform better. Dunno.n
A GPS speedo is not reliable at very low speeds. When stationary it can still register speeds, sometimes quite high. It's just its inherent randomness that you can't do much about except filter out spurious values. But for showing your car speed while actually driving, I find it does a fine job.
If distance measuring is important then an HE sensor counting wheel clicks can't be beat, don't you think?